JPH026670Y2 - - Google Patents

Abstract

An acoustic wave filter comprising a piezoelectric substrate body upon which at least two digital structures are formed and which comprise input and output transducers and form a selective wave reflective pair with at least one of the digital structures having a wide band amplitude characteristic F1 and at least one of the other digital structures having a narrower frequency band amplitude characteristic and wherein the center frequency f1 of the wider band F1 digital structure is displaced relative to the center frequency f2 of the digital structure which has a narrower frequency band F2 by a frequency of DELTA f on either side of the center frequency f1 so that one of the zero locations fR1'2 of the distortion signal FR of interdigital reflections is placed close to the value of the center frequency f0 of the filter.

Description

[Detailed explanation of the idea] [Industrial application field] The present invention relates to acoustic surface acoustic wave devices.

[Conventional technology]

At least two interdigital structures with a predetermined amplitude-frequency characteristic F ₀ and a predetermined center frequency ₀ are provided on the piezoelectric substrate as input and output transducers, optionally in the case of resonators. They are arranged as a pair of wave reflectors, with at least one interdigital structure having a center frequency ₁ and a broadband amplitude frequency characteristic F ₁ and at least the other interdigital structure having a center frequency ₂ ≈ _0.
Acoustic surface acoustic wave devices are known which have a narrow band amplitude frequency characteristic F ₂ ≈F ₀ compared to the amplitude frequency characteristic F ₁ .

Such an acoustic surface acoustic wave device can be a delay device with waves propagating in one direction in a filter, or a resonator with acoustic waves inside.

For example, “Electronics Letters” 1973, 9 volumes,
98MHz as known from pages 195-197, especially fig.
Oscillators include the devices handled in this invention. The device comprises a relatively broadband interdigital structure of length r·λ (where r is the logarithm of the interdigital fingers forming the interdigital structure and λ is the wavelength);
In comparison, it is narrow-band and includes an interdigital structure of length P·λ (r is the logarithm of the interdigital fingers forming the interdigital structure, λ is the wavelength). This narrow band interdigital structure is divided into several sections when viewed in the longitudinal direction, a predetermined interdigital strip (finger) is provided in a part of each section, and an interdigital strip is installed between the sections. There are parts without. This is because the provision of interdigital strips over the entire length of the interdigital structure is on the one hand not required for narrowband properties, and on the other hand involves high insertion attenuation of the device. Both interdigital structures have the same predetermined intermediate frequency.
Has ₀ . (Transducer) The reason why the interdigital structure is not completely filled with interdigital strips, i.e. not provided with interdigital strips over its entire length, is to prevent signal interference due to reflections in the interdigital structure of the device. The idea is to reduce it. Such reflection is based on, for example, a wave emitted by reflection in the opposite direction from the narrowband interdigital structure p and λ, and this wave hits the broadband interdigital structure r and λ, and is reflected from this structure. is reflected again on the interdigital strip and reaches the narrowband structure again as a delayed signal. The resulting signal is relatedly referred to as a "triple transit" signal. The appearance of such a disturbance signal leads to a so-called "ripple" in the amplitude characteristic curve F ₀ of the device.

In addition to the above-mentioned measures that do not completely fill the transducer structure, various further measures have already been proposed for suppressing signals that arise due to undesired multiple reflections in the interdigital structure (triple transitions). is being carried out. For example, means are used to divide the interdigital strips of one structure into two electrically interconnected parallel strips (dividing fingers) each having a half width. However, since the maximum allowable width of an interdigital strip (or finger) is determined by the device's predetermined frequency or band center frequency _{of 0} , the use of the "split finger" principle can result in extremely narrow interdigital strips. This would lead to stricter requirements regarding the fabrication of filters, or to reduce the maximum frequency that the filter can be designed for by a factor of two.

[Problem that the invention attempts to solve]

The purpose of the present invention is to obtain a surface acoustic wave device that can suppress signals based on reflections in interdigital structures without increasing technical difficulties, and without changing the maximum achievable frequency. .

[Means for solving problems]

This object, according to the invention, comprises a piezoelectric substrate having a predetermined amplitude-frequency characteristic F ₀ and a predetermined center frequency ₀ and having at least two interdigital structures, the at least one interdigital structure being The at least one other interdigital structure has a center frequency ₁ and a broadband amplitude-frequency characteristic F ₁ , and the at least one other interdigital structure has a center frequency ₂ ≈ ₀ and a narrow-band amplitude-frequency characteristic F ₂ with respect to said broadband amplitude-frequency characteristic F _{1 .} ≈ ₀ , the broadband interdigital structure has at least twice the bandwidth of the narrowband interdigital structure, and for said predetermined center frequency ₀ ≈ ₂ , the center frequency ₁ is ₁ . = ₂ /1±1/2N ₁ -1, where N ₁ is the predetermined center frequency
This is achieved by representing the number of wavelengths of surface acoustic waves entering the broadband interdigital structure in the longitudinal direction at ₀ .

In addition to achieving the above-mentioned objects, the invention also provides the possibility that in an interdigital structure fully filled with interdigital strips, interference signals appear which are more or less suppressed due to the low degree of filling. Therefore, the minimum value of insertion attenuation is obtained.

As will be explained later in detail with reference to the figures, the present invention provides that the amplitude-frequency characteristic curve F _R of a disturbance signal based on interdigital reflections, especially in a broadband interdigital structure, has zero positions _R1 and _R2 below and above its center frequency. It starts from the recognition or confirmation that one has. These zero positions _R1 and _R2
lies within the main maximum of the amplitude-frequency characteristic curve F ₁ of the broadband (transducer) interdigital structure.

In the present invention, based on the above recognition, the center frequency ₁ of the interdigital structure having the broadband amplitude frequency characteristic curve F ₁ is changed by the value of the frequency Δ,
With respect to a predetermined center frequency ₀ , and hence the center frequency ₂ of the narrowband interdigital structure, the center frequency ₁ of the broadband interdigital structure is shifted as follows: One of the zero positions _{, R1} or _R2 , is shifted so that it at least sufficiently coincides with the frequency value ₀ of the device. According to the invention, a correspondingly reduced bandwidth of the narrowband structure by at least a factor of at least twice, in particular by a factor of at least five, ensures that at most only a negligible amplitude component of the interference signal F _R Transmission band of the device by F ₀
to ensure that only the contents are included.

〔Example〕

Next, an embodiment of the present invention shown in the drawings will be described. The first example shows a device according to the invention configured as a narrowband delay line, and the second example shows a device according to the invention as a resonator.

FIG. 1 shows the structure of a delay line according to the invention, in which a delay line 11 has, for example, a first interdigital structure 12 as an input converter and a second interdigital structure 13 as an output converter. Both of these interdigital structures 12,1
3 is present on a piezoelectric substrate 14 made of quartz or lithium niobate, for example. Based on its length p·λ, the interdigital structure 13 has a relatively narrow frequency band with a center frequency ₂ , that is, a narrow frequency characteristic curve F ₂ . The individual interdigital strips (or fingers) of interdigital structures 12, 13 are shown simply as lines 17 in the figure. In reality these strips are approximately 500MHz
It has a width of 2 μm for the maximum frequency of .
The length of the individual interdigital strips 17 is determined by the signal power to be transmitted and can be, for example, 1 mm. Interdigital structure part 1
2, 13, the interdigital strips 17 each alternately connect the connecting bridge 1.
21, 122 or 131, 132 are electrically connected.

Figure 1 shows only the important parts of the embodiment of the present invention, in addition to which it also shows the terminal wires to be connected to the connecting bridge, the acoustic damper provided at the edge of the board, and other individual parts not shown. can be added.

Assuming that the interdigital structure 12 is used as an input transducer, the substrate 1 is
The acoustic surface waves generated piezoelectrically at 4 (represented by the dashed line 15) are propagated in the direction of the arrow 16 into the region of the interdigital structure 13 of the output transducer, where they are again piezoelectrically converted back into an electrical signal wave. Ru.

FIG. 4 shows a diaphragm of amplitude-frequency characteristics of a device according to the prior art, in which the horizontal axis represents frequency and the vertical axis represents amplitude. More specifically, the amplitude-frequency characteristic curve F ₁ for the broadband interdigital structure 12, and an amplitude frequency characteristic curve F _R (this will be explained in more detail later) for the interdigital reflected signal (triple transit signal, TTS) in the broadband interdigital structure 12. Amplitude frequency characteristic curve
The mutual frequency relationship of F ₁ and F ₂ is the center frequency ₂ of the narrowband interdigital structure 13.
corresponds at least approximately to the center frequency ₁ of the broadband interdigital structure 12. The resulting total amplitude-frequency characteristic curve is based on the mutual multiplication of both amplitude-frequency characteristic curves F ₁ and F ₂ (here with sufficient accuracy for appropriate considerations and the main maximum of the amplitude-frequency characteristic curve F ₁ ). (within values), approximately corresponds to the amplitude-frequency characteristic curve F ₂ of a narrowband interdigital structure. Therefore, the total delay line 11
The center frequency ₀ of is approximately equal to the center frequency ₂ of the amplitude frequency characteristic curve F ₂ . The ripple 21 shown in the amplitude-frequency characteristic curve F ₁ is due to the superposition of the interdigital reflection signal F _R (TTS-signal). In this way, the total amplitude frequency characteristic curve F ₀ of the delay line 11 has 22 parts in the amplitude frequency characteristic curve F _{2 .}
It has a disturbing effect as shown by the broken line. The desired linearity phase characteristic with respect to frequency for the device is:
They will be disturbed accordingly.

As already mentioned, for the amplitude-frequency characteristic curve F _R of the signal of the interdigital reflection in the (transducer) interdigital structure 12, considered separately here, the course designated F _R is given. It can be seen that the two first minima F _R1 , _R2 lie within the main maximum 23 of the amplitude-frequency characteristic curve _F 1 of the broadband interdigital structure. As shown in the figure, the minimum values _R1 and _R2 are
Both are half the distance between the center frequency ₁ of the amplitude-frequency characteristic curve F ₁ and the respective first zero position of this amplitude-frequency characteristic curve F ₁ . The amplitude frequency characteristic curve F ₁ of the broadband interdigital structure 12 is
At these frequency values _R1 and _R2 , the amplitude frequency characteristic curve of the narrow band interdigital structure section 13
The bandwidth ratio is at least 2:1, i.e. as important for the application as compared to the bandwidth of F ₂ ,
In particular, when the ratio is 5:1 or more, there is still no significant drop.

Next, the present invention will be explained with reference to FIG. In FIG. 3, the amplitude-frequency characteristic curve of the signal of the interdigital reflection in a broadband interdigital structure with zero positions _R1 , _R2 is likewise designated by F _R. The amplitude frequency characteristic curve F ₁ is shown without the ripple 21 shown for the prior art in FIG. 4 but eliminated by the present invention. Corresponding to the frequency shift Δ provided by the present invention,
The amplitude frequency characteristic curve (indicated by a solid line) F ₂ (≈F ₀ ) of the narrowband interdigital structure 13 is drawn so that its center frequency ₂ is located at the frequency _R1 . As an equivalent solution, the dashed curve shows an amplitude-frequency characteristic curve F ₂ ′ with center frequency ₂ ′ at frequency _R2 . For both solutions, one zero position of the amplitude-frequency characteristic curve F _R coincides with the center frequency ₂ ( ₂ ') of the narrowband interdigital structure, and the amplitude-frequency characteristic curve F ₁ coincides with the center frequency 2 (2') of the narrowband interdigital structure. Since the amplitude frequency characteristic curve F ₂ of the structural part 13 does not have a significant amplitude drop over the main maximum band, the total amplitude frequency characteristic curve F ₀
Corresponding to the problem underlying the invention, the amplitude disturbances as indicated by the dashed line 22 no longer appear at the maximum value of the amplitude-frequency characteristic curve F ₂ in FIG. The amplitude-frequency characteristic curve F ₀ is for the main maximum value,
This corresponds to a quantitative change in the amplitude-frequency characteristic curve F ₂ of the narrowband interdigital structure 13 . The sub-maximum does not have significant asymmetric damping due to the asymmetric variation of the amplitude-frequency characteristic curve F ₁ with respect to the center frequency ₂ .

Therefore, in the present invention, from a phenomenological perspective, it is possible to definitively prevent the sound waves emitted from the narrowband interdigital structure 13 from being reflected in the broadband interdigital structure 12, thereby preventing interference. The triple-transit signal no longer appears.

FIG. 3 shows an embodiment of the present invention as a resonator. The resonator 40 has structures 41 and 141 and structures 42 and 43, the interdigital strips of which are likewise connected to the piezoelectric substrate 4.
Exists on 4. Interdigital structure section 42
It is assumed that is an input converter and interdigital structure 43 is an output converter. As in the embodiment of FIG. 1, the input and output transducers can be interchanged here as well. Furthermore, in the resonator of FIG. 3, the same structure 42 or 43 is
It can be an input transducer as well as an output transducer. This is primarily a problem with the peripheral connections of the resonator.

The interdigital structures 41 and 141 shown in FIG. 3 together form a reflector pair which defines an acoustic resonator for the sound wave indicated at 15 in this resonator. A standing acoustic wave is formed between these two structures 41 and 141, which is indicated by the double-headed arrow 46. As is known, the wavelength of the waves present in the resonator, i.e. the magnitude of the wavelength corresponding to the resonant frequency of the resonator, is determined by both reflector structures 4.
This is determined by the selection of the periodicity of the strips 1 and 141 and the spacing between the two structures 41, 141.
This spacing is decisive for the degree of reflection quality of both structures 41, 141 as well as for the quality of the passive resonator cavity formed from these structures 41, 141. The inverse value of this quality is a measure for the fractional bandwidth of the amplitude-frequency characteristic curve F ₂ of the total interdigital structure formed from the good interdigital structure 41, 141 of the reflector pair.
especially compared to the dimensions of the remainder of the structure in general.
Interdigital structure 4 forming a reflector
1,141 Due to their large mutual spacing, this reflector pair 41,141 forms a structure with a narrow-band amplitude-frequency characteristic curve F ₂ (and phase characteristic curve) of the resonator. In other words, in this configuration of the filter according to the invention as a resonator, the two interdigital structures 41, 141 together form a single interdigital structure, each consisting of a number of interdigital strips. Formed by the gist of the claims. The structures 41, 141 forming this resonator do not require electrical terminals, as is known. These structures are therefore constructed not only as metal strips provided on the surface of the substrate 44, but also as corresponding strip-like surface depressions or grooves in the substrate, or strip-like elevations on the substrate surface. This can also be achieved by

This amplitude-frequency characteristic curve F ₂ of the reflector pair 41, 141 has a center frequency ₂ . The reflector pair 41, 141 here corresponds to the narrowband interdigital structure 13 of FIG. As in the delay line according to _FIG.
and its center frequency ₂ is determined by the input transducer and/or output transducer 42 and/or 43
at least twice, especially at least 5 times compared to
For a twice narrower bandwidth, determine the bandwidth F ₀ and center frequency ₀ of the entire resonator in FIG.

The following formula is given mathematically for the present invention.

₁ = ₀ /1±1/2N ₁ -1 where ₀ is the predetermined center frequency of the device and ₁ is the center frequency to be chosen according to the invention of the broadband interdigital structure. This formulation also includes the summation of the broadband (transducer) interdigital structure 12, or of both structures 42, 43 in the resonator embodiment of FIG. Enter the total length l. That is, since N ₁ is the number of wavelengths of surface acoustic waves entering the interdigital structure in the longitudinal direction, if the wavelength is λ, then l=N ₁ ·λ/2.
In the latter case, the basis is the total length l from the first interdigital strip on the left of structure 42 to the last interdigital strip on the right of structure 43. What happens to these two structural parts 42 and 43?
act together like a single structure on the interdigital reflection F _R of the interference signal to be removed, and this single structure corresponds to the intermediate space between the two structures 42, 43. This is because it is not completely filled there by the interdigital strip. Associated with this length is the number N ₁ of the acoustic wavelengths of the surface waves that are adapted to each of these length dimensions.
For both prefixes + and -, _R1 according to the present invention
We get a shift from to ₂ and from _R2 to ₂ .

In the present invention, after the interference signals due to interdigital reflections that would otherwise appear are removed, in a device constructed according to the present invention, the interference signals due to unwanted reflections of sound waves at the left or right edge of the respective substrates are removed. Jamming signals may appear. In order to eliminate such reflections, it is recommended to configure the substrates with a slope at each end, as can be seen in the figure. This is usually done in practice, especially for delay devices (FIG. 1).

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the device of the present invention as a delay line, FIG. 2 is a diagram of the amplitude frequency characteristic curve for the device according to FIG. 1, and FIG.
The figure shows an embodiment of the device of the present invention as a resonator, and FIG. 4 is a diagram of an amplitude frequency characteristic curve for a device according to the known technology. DESCRIPTION OF SYMBOLS 11... Delay line, 12, 13... Interdigital structure part, 14... Piezoelectric substrate, 15... Wave line, 16... Wave propagation direction, 17... Interdigital strip, 40... Resonator, 41 ,141...
Interdigital structure (reflector pair), 42,
43... Interdigital structure section, 44... Piezoelectric substrate, 46... Wave propagation direction, F ₀ , F ₁ , F ₂ ,
F _R ...Amplitude frequency characteristic curve, ₀ , ₁ , ₂ ...Center frequency.

Claims (1)

[Claims for Utility Model Registration] 1. At least two interdigital structures 12, 13, 42, 43, 41, having a predetermined amplitude frequency characteristic F ₀ and a predetermined center frequency ₀ ;
comprising a piezoelectric substrate 14, 44 having 141;
at least one interdigital structure 1
2, 42, 43 have a center frequency ₁ and a broadband amplitude frequency characteristic F ₁ , and at least one other interdigital structure 13, 41, 14
1 is the center frequency ₂ ≒ ₀ and the narrow band amplitude frequency characteristic F ₂ ≒ compared to the wide band amplitude frequency characteristic F ₁ .
In a surface acoustic wave device having F ₀ , broadband interdigital structures 12, 42,
43 is a narrowband interdigital structure section 1
3, 41, 141, and for the predetermined center frequency ₀ ≈ ₂ , the center frequency ₁ has a relationship of ₁ = ₂ / 1 ± 1 / 2N ₁ -1, where N ₁ is the given center frequency
A surface acoustic wave device characterized in that ₀ represents the number of wavelengths of surface acoustic waves entering the broadband interdigital structure 12; 42, 43 in the longitudinal direction. 2 Broadband interdigital structure section 12, 4
Utility model registration claim ₁ , characterized in that the amplitude frequency characteristic F ₁ of the narrowband interdigital structure 13; The surface acoustic wave device described. 3 Utility model registration claim 1 characterized in that the narrow band interdigital structure is constituted by an interdigital strip 17
The surface acoustic wave device according to item 1 or 2. 4 Two interdigital structures 12, 13
, one of which is an input transducer and the other is an output transducer, according to any one of claims 1 to 3 of the utility model registration claim. . 5. Utility model registration claim 1 or 2, characterized in that the narrow band interdigital structure is formed by a pair of reflectors 41, 141 each having an interdigital strip and is used as a resonator. The surface acoustic wave device described. 6. The surface acoustic wave device according to claim 5, wherein the output transducer and the input transducer are formed of interdigital structures 42 and 43 having the same structure and are used as a resonator.